During Evening mass (generally 7:00 PM to 7:30 PM)
Location: Confessional rooms within the church (The green light above the confessional door signifies that the priest is present inside the confessional room)
What is & Why confession?
CCC 1422 "Those who approach the sacrament of Penance obtain pardon from God's mercy for the offense committed against him, and are, at the same time, reconciled with the Church which they have wounded by their sins”..... Bible/Early Church Refrences Link
Why to a Bishop/Priest only?
CCC 1548 "the priest of the old law was a figure of Christ, and the priest of the new law acts in the person of Christ" (i.e. Having the indwelling Holy Spirit, they act in the person of Christ exercising Christ’s Sanctifying Role)
CCC 1461 Since Christ entrusted to his apostles the ministry of reconciliation*. Bishops who are their successors, and priests, the bishops' collaborators, continue to exercise this ministry.
* John 20: 21-23 “Jesus said to them again, "Peace be with you. As the Father has sent me, even so I send you." And when he had said this, he breathed on them, and said to them, "Receive the Holy Spirit. If you forgive the sins of any, they are forgiven; if you retain the sins of any, they are retained."
Acts necessary for the penitent (Sorrow, Confession & Penance):
CCC 1451: Among the penitent's acts contrition occupies first place. Contrition is "sorrow of the soul and detestation for the sin committed, together with the resolution not to sin again."
CCC 1456 Confession to a priest is an essential part of the sacrament of Penance: All mortal sins of which penitents after a diligent self-examination are conscious must be recounted by them in confession, even if they are most secret.
CCC 1459 Many sins wrong our neighbor. One must do what is possible in order to repair the harm (e.g., return stolen goods, restore the reputation of someone slandered, pay compensation for injuries). Simple justice requires as much. He/She must "make satisfaction for" or "expiate" his sins. This satisfaction is also called "penance." that the priest imposes.
When & How often?
CCC 1457 According to the Church's command, "after having attained the age of discretion, each of the faithful is bound by an obligation faithfully to confess serious sins at least once a year." Anyone who is aware of having committed a mortal sin must not receive Holy Communion
CCC 1458 Without being strictly necessary, confession of everyday faults (venial sins) is nevertheless strongly recommended by the Church.
CCC 1483 In the case of the General Absolution to be valid the faithful must have the intention of individually confessing their grave sins in the time required
CCC=Catechism of the Catholic Church